How intracellular pathogens such as Legionella pneumophila are eliminated from the lung is unknown. The mechanisms underlying parasitism of alveolar macrophages (AM), the interactions of cytokines mediating the immunoregulatory and effector cell functions of AM, and the relative contributions of macrophage activation and cytotoxic responses to the resolution of intracellular infection in the lung are unknown. The growing population of immunocompromised individuals susceptible to opportunistic infection underscores the importance of developing new strategies for the prevention and treatment of intracellular infections. The development of effective vaccines and immunotherapy is dependent on an understanding of the protective host response. The long-term objective of this proposal is to define molecular and cellular elements of the protective host responses to intracellular infections of the lung and to identify potential approaches to the augmentation of host resistance by combining in vitro work with human AM and in vivo studies in mice with specific cytokine deficiencies. The proposed studies are based on the following hypotheses: 1) The survival of L. pneumophila in human AM is governed by the balance of cytokines that stimulate or inhibit cellular resistance to infection by regulating bacterial uptake and the availability of intracellular iron. Virulent organisms manipulate the cytokine response to favor their intracellular survival. 2) The initial interaction of human AM with L. pneumophila is mediated by pattern recognition receptors such as CD14, that mediate bacterial binding and the cytokine response to L. pneumophila LPS and whole bacteria. 3) INF- gamma and TNF-alpha are required for the resolution of intracellular infection of the lung, as essential signals for the activation of AM and the destruction of infected cells by natural killer (NK) cells and cytotoxic lymphocytes (CTL). The specific aims are: 1) to define cytokine profiles that are permissive of or protective against intracellular infection of human AM by L. pneumophila, and establish whether virulent organisms manipulate the responses of AM to favor their intracellular survival: 2) to determine whether pattern recognition receptors such as CD14 mediate bacterial binding and the cytokine response of human AM to L. pneumophila LPS and whole bacteria; and 3) to delineate the relative contribution of macrophage activation and cytotoxic responses in the resolution of pneumonic legionellosis through the comparative study and selective reconstitution of transgenic mice deficient in INF-gamma or TNF-alpha.